What's your in-game resolution? Are you using the latest Geforce drivers?
WoW has always been heavily CPU-bound and while your PC specs are orders of magnitude better than what was available when WoW was released in 2004 there are a few things to keep in mind. Firstly, Nostlarius is using the MANGOS core (i.e. to emulate the vanilla server side software stack) and the server hosts significantly more players than a vanilla server would have allowed back in 2004/2005. When WoW launched most players would have been playing at 800x600 to 1024x768 on a single core Intel Pentium 4 or Athlon XP with 512MB - 1GB of RAM. You would have needed a top-end rig to play at 1600x1200 with some AA back then.
I've played on a bunch of private servers and found that vanilla mangos servers seem to have the biggest performance impact on the same hardware relative to other emulations and retail WoW. I upgraded to an X99 system a few months ago but before that was running a 2.8GHz AMD Phenom II X6, 8GB RAM and a 3GB Radeon 280x with a 16:10 1920x1200 monitor. On retail WoW (well until mid-MoP when I finally quit

), Atlantiss (Cata private server), Dalaran (WotLK private server), and Excalibur (TBC private server) I'd typically get 80+ fps at 1920x1200 with max settings and 4x AA. However, on the Rebirth (vanilla private server) and Nostalrius I was getting fluctuating fps from 30-80+fps depending on the area. In IF on Nostalrius my fps could drop to 5-15 during peak times. I suspect that part of that was due to the Radeon drivers being optimised for more recent DX11 iterations of WoW rather than the DX9 path in vanilla. When I played on the Rebirth I had to revert to some fairly old Catalysts because more recent versions were causing graphics corruption in-game.
Ironforge and Orgrimmar at peak times can tax even a high end system today depending on the settings you play at because of the numbers of players in a condensed area. Even on my current rig (X99 5820k, 32GB RAM, 6GB Geforce 980GTX @ 2560x1440) I get fps drops down to 40-45s at times in Ironforge during peak times although it's otherwise always capped at 60fps (V-sync on).
You shouldn't really be getting sub 40fps with your rig in a 40-man raid though. It could be an errant addon causing slowdown or fixable by toning down a quality setting. While your CPU is still alright it's over 5 years old and depending on your other settings and how many other apps and programmes (e.g. Skype, Twitch, Chrome with 20+ tabs open, etc) it will probably struggle in high population areas like IF.
TLDR: Upgrade or overclock your CPU.